Non-volatile memory cells, memory arrays including the same and methods of operating cells and arrays

ABSTRACT

Memory cells comprising: a semiconductor substrate having a source region and a drain region disposed below a surface of the substrate and separated by a channel region; a tunnel dielectric structure disposed above the channel region, the tunnel dielectric structure comprising at least one layer having a small hole-tunneling-barrier height; a charge storage layer disposed above the tunnel dielectric structure; an insulating layer disposed above the charge storage layer; and a gate electrode disposed above the insulating layer are described along with arrays thereof and methods of operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon, and claims priority under 35 U.S.C. §119(e) of: provisional U.S. Patent Application No. 60/640,229, filed onJan. 3, 2005; provisional U.S. Patent Application No. 60/647,012, filedon Jan. 27, 2005; provisional U.S. Patent Application No. 60/689,231,filed on Jun. 10, 2005; and provisional U.S. Patent Application No.60/689,314, filed on Jun. 10, 2005; the entire contents of each of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

Non-volatile memory (“NVM”) refers to semiconductor memory which is ableto continually store information even when the supply of electricity isremoved from the device containing the NVM cell. NVM includes MaskRead-Only Memory (Mask ROM), Programmable Read-Only Memory (PROM),Erasable Programmable Read-Only Memory (EPROM), Electrically ErasableProgrammable Read-Only Memory (EEPROM), and Flash Memory. Non-volatilememory is extensively used in the semiconductor industry and is a classof memory developed to prevent loss of programmed data. Typically,non-volatile memory can be programmed, read and/or erased based on thedevice's end-use requirements, and the programmed data can be stored fora long period of time.

Generally, non-volatile memory devices may have various designs. Oneexample of an NVM cell design is the so-called SONOS(silicon-oxide-nitride-oxide-silicon) device, which may use a thintunnel oxide layer, to allow hole direct tunneling erase operations.Although such designs may have good erase speed, the data retention isusually poor, in part because direct tunneling may occur even at a lowelectrical field strengths that may exist during a retention state of amemory device.

Another NVM design is NROM (nitrided read-only memory), which uses athicker tunnel oxide layer to prevent charge loss during retentionstates. However, a thick tunnel oxide layer may impact channel erasespeed. As a result, band-to-band tunneling hot-hole (BTBTHH) erasemethods can be used to inject hole traps to compensate the electrons.However, the BTBTHH erase methods may cause some reliability issues. Forexample, the characteristics of NROM devices employing BTBTHH erasemethods may degrade after numerous P/E (program/erase) cycles.

Thus, a need in the art exists for non-volatile memory cell designs andarrays which can be operated (programmed/erased/read) numerous timeswith improved data retention performance and increased operation speeds.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to non-volatile memory devices, and morespecifically, to non-volatile memory devices including a tunneldielectric structure that facilitates self-converging erase operationswhile also maintaining charge retention in a charge storage layer of thememory device during retention states.

One embodiment of the present invention includes memory cellscomprising: a semiconductor substrate having a source region and a drainregion disposed below a surface of the substrate and separated by achannel region; a tunnel dielectric structure disposed above the channelregion, the tunnel dielectric structure comprising at least one layerhaving a small hole-tunneling-barrier height; a charge storage layerdisposed above the tunnel dielectric structure; an insulating layerdisposed above the charge storage layer; and a gate electrode disposedabove the insulating layer.

Another embodiment of the present invention includes memory cellscomprising: a semiconductor substrate having a source region and a drainregion disposed below a surface of the substrate and separated by achannel region; a multi-layer tunnel dielectric structure disposed abovethe channel region, the multi-layer tunnel dielectric structurecomprising at least one layer having a small hole-tunneling-barrierheight; a charge storage layer disposed above the multi-layer tunneldielectric structure; an insulating layer disposed above the chargestorage layer; and a gate electrode disposed above the insulating layer.

In certain preferred embodiments, the layer providing a smallhole-tunneling-barrier height may contain materials such as siliconnitride (Si₃N₄) or hafnium oxide (HfO₂). In certain preferredembodiments of the present invention memory cells include a tunneldielectric structure having multiple layers, such as a stackeddielectric tri-layer structure of silicon oxide, silicon nitride, andsilicon oxide (ONO). Such tunnel dielectric structures provide a SONONOS(silicon-oxide-nitride-oxide-nitride-oxide-silicon) or a super-latticeSONONOS design.

In certain preferred embodiments of the present invention the tunneldielectric structure can comprise at least two dielectric layers eachhaving a thickness of up to about 4 nm. Additionally, in certainpreferred embodiments of the present invention, the gate electrodecomprises a material having a work function value greater than that ofN⁺ polysilicon.

In certain preferred embodiments, the tunnel dielectric structure caninclude a layer comprising a material having a small hole tunnelingbarrier height, wherein the material is present in the layer at aconcentration gradient such that the concentration of the material is ata maximum at a depth point within the layer.

The present invention also includes non-volatile memory devices whichcomprise a plurality of memory cells (i.e., an array) in accordance withone or more of the embodiments described herein. As used herein, a“plurality” refers to two or more. Memory devices in accordance with thepresent invention exhibit significantly improved operational propertiesincluding increased erase speeds, improved charge retention and largerwindows of operation.

The present invention also includes methods of operating non-volatilememory cells and arrays. Methods of operation in accordance with thepresent invention include resetting the memory devices by applying aself-converging method to tighten Vt distribution of the memory devices;programming at least one of the memory devices by channel +FN injection;and reading at least one of the memory devices by applying a voltagebetween an erased state level and a programmed state level of at leastone of the memory devices. As used herein, the term “tighten” refers tothe narrowing of the threshold voltage distribution among the manymemory cells of an array. In general, threshold voltage distribution is“tightened” where the threshold voltages of several cells are within anarrow range of one another such that operation of the array is improvedover conventional designs. For example, in some preferred embodiments,such as in a NAND array comprising memory cells in accordance with oneor more embodiments of the present invention, a “tightened” thresholdvoltage distribution indicates that the threshold voltages of thevarious memory cells are within a 0.5V range of one another. In otherarray architectures employing memory cells in accordance with thepresent invention, the “tightened” threshold voltage distribution mayhave a range of about 1.0V from the upper limit to the lower limit.

One embodiment of a method of operation in accordance with the presentinvention includes operating an array in accordance with the presentinvention by applying self-converging reset/erase voltages to thesubstrate and the gate electrode in each memory cell to be reset/erased;programming at least one of the plurality of memory cells; and readingat least one of the plurality of memory cells by applying a voltagebetween an erased state level and a programmed state level of at leastone of the memory devices.

The present invention also includes methods of forming a memory cell,comprising: providing a semiconductor substrate having a source regionand a drain region formed therein below a surface of the substrate andseparated by a channel region; forming a tunnel dielectric structureabove the channel region, wherein forming the tunnel dielectricstructure comprises forming at least two dielectric layers, wherein oneof the at least two dielectric layers has a smaller hole tunnelingbarrier height than the other of the at least two dielectric layers;forming a charge storage layer above the tunnel dielectric structure;forming an insulating layer above the charge storage layer; and forminga gate electrode above the insulating layer.

As used herein, the phrase “small hole tunneling barrier height” refersgenerally to values which are less than or equal to the approximate holetunneling barrier height of silicon dioxide. In particular, a small holetunneling barrier height is preferably less than or equal to about 4.5eV. More preferably, a small hole tunneling barrier height is less thanor equal to about 1.9 eV.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown.

In the drawings:

FIGS. 1 a and 1 b are cross-sectional schematic representations of anN-channel memory cell in accordance with one embodiment of the presentinvention and a P-channel memory cell in accordance with one embodimentof the present invention, respectively;

FIG. 2 is a graphical representation of the threshold voltage (chargetrapping capacity) of a tunnel dielectric structure in accordance withone embodiment of the present invention under various programmingmethods;

FIG. 3 is a graphical representation of the threshold voltage of aSONONOS memory cell in accordance with one embodiment of the presentinvention over time during erase;

FIG. 4 is a graphical representation of the threshold voltage of aSONONOS memory cell in accordance with one embodiment of the presentinvention over time during retention;

FIGS. 5 a–5 e are band energy diagrams of ONO tunnel dielectricstructures in accordance with various embodiments of the presentinvention;

FIG. 6 is a graphical representation of hole-tunneling current versuselectrical field strength for three different tunnel dielectricstructures;

FIG. 7 a is a graphical representation of the threshold voltage overtime of a memory cell in accordance with one embodiment of the presentinvention during erase after various types of programming;

FIG. 7 b is a graphical representation of the threshold voltage overtime of a memory cell having a platinum gate in accordance with oneembodiment of the present invention during erase;

FIGS. 7 c and 7 d are graphical representations of capacitance versusvoltage for the memory cell referred to in FIG. 7 b;

FIG. 8 is a graphical representation of the threshold voltage of amemory cell in accordance with one embodiment of the present inventionover the course of numerous programs/erase cycles under variousoperating conditions;

FIG. 9 is a graphical representation of the current-voltage (IV)relationship for a memory cell in accordance with one embodiment of theinvention after one cycle and 10³ cycles;

FIG. 10 is a graphical representation of the threshold voltage of amemory cell in accordance with one embodiment of the present inventionover the course of numerous programs/erase cycles under one set ofprogramming and erasing conditions;

FIG. 11 is a graphical representation of the change in threshold voltageover time in a memory cell according to one embodiment of the presentinvention under VG-accelerated retention testing;

FIGS. 12 a and 12 b are an equivalent circuit diagram and layout view,respectively, of a virtual ground array of memory cells in accordancewith one embodiment of the present invention;

FIG. 13 is a cross-section schematic representation of a virtual groundarray of memory cells in accordance with one embodiment of the presentinvention taken along line 12B—12B as shown in FIG. 12 b;

FIGS. 14 a and 14 b are equivalent circuit diagrams of memory arrayscomprising memory cells in accordance with one embodiment of the presentinvention and depicting suitable reset/erase voltages in accordance withtwo embodiments of operation in accordance with the present invention;

FIGS. 15 a and 15 b are equivalent circuit diagrams of memory arrayscomprising memory cells in accordance with one embodiment of the presentinvention depicting one method of programming in accordance with thepresent invention;

FIGS. 16 a and 16 b are equivalent circuit diagrams of memory arrayscomprising memory cells in accordance with one embodiment of the presentinvention depicting one method of reading a bit in accordance with thepresent invention;

FIG. 17 is a graphical representation of the threshold voltage of amemory cell in accordance with one embodiment of the present inventionover time under various erasing conditions;

FIG. 18 is a graphical representation of the threshold voltage of amemory cell in accordance with one embodiment of the present inventionover the course of numerous programs/erase cycles;

FIGS. 19 a and 19 b are graphical representations of the current at thedrain of a memory cell in accordance with one embodiment under variousgate voltages depicted in a logarithmic scale and a linear scale,respectively;

FIG. 20 is an equivalent circuit diagram of an array including memorycells in accordance with one embodiment of the present inventiondepicting one method of programming a bit in accordance with the presentinvention;

FIGS. 21 a and 21 b are a layout view and equivalent circuit diagram ofa virtual ground array in accordance with one embodiment of the presentinvention;

FIGS. 22 a and 22 b are an equivalent circuit diagram and layout view,respectively, of a NAND array of memory cells in accordance with oneembodiment of the present invention;

FIGS. 23 a and 23 b are cross-sectional schematic representations of aNAND array of memory cells in accordance with one embodiment of thepresent invention taken along lines 22A—22A and 22B—22B, respectively,as shown in FIG. 22 b;

FIG. 24 a is an equivalent circuit diagram of a NAND array in accordancewith one embodiment of the present invention depicting one method ofoperation in accordance with the present invention;

FIG. 24 b is a graphical representation of threshold voltages over timeduring a reset operation in accordance with one embodiment of thepresent invention for two memory cells having different initialthreshold voltages;

FIG. 25 is an equivalent circuit diagram depicting a method of operationin accordance with one embodiment of the present invention;

FIG. 26 is a graphical representation of the threshold voltage of amemory cell in accordance with one embodiment of the present inventionover time under various erasing conditions;

FIG. 27 is an equivalent circuit diagram depicting a method of operationin accordance with one embodiment of the present invention;

FIG. 28 is a graphical representation of the threshold voltage of amemory cell in accordance with one embodiment of the present inventionover the course of numerous programs/erase cycles under one set ofprogramming and erasing conditions

FIGS. 29 a and 29 b are graphical representations of the current at thedrain of a memory cell in accordance with one embodiment under variousgate voltages at three different cycle numbers depicted in a logarithmicscale and a linear scale, respectively

FIG. 30 is a graphical representation of the threshold voltage of memorycells in accordance with one embodiment of the present invention overtime during retention at three different temperature and cycleconditions;

FIG. 31 is a cross-sectional schematic representation of a NAND arraywordline in accordance with one embodiment of the present invention; and

FIG. 32 is a cross-sectional schematic representation of a NAND arraywordline formation technique in accordance with one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the invention and the presentlypreferred embodiments thereof, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same or similar referencenumbers are used in the drawings and the description to refer to thesame or like parts. It should be noted that the non-graph drawings arein greatly simplified form and are not to precise scale. In reference tothe disclosure herein, for purposes of convenience and clarity only,directional terms, such as top, bottom, left, right, up, down, above,below, beneath, rear, and front, are used with respect to theaccompanying drawings. Such directional terms used in conjunction withthe following description of the drawings should not be construed tolimit the scope of the invention in any manner not explicitly set forthin the appended claims. Although the disclosure herein refers to certainillustrated embodiments, it is to be understood that these embodimentsare presented by way of example and not by way of limitation. It is tobe understood and appreciated that the process steps and structuresdescribed herein do not cover a complete process flow for themanufacture of entire integrated circuits. The present invention may bepracticed in conjunction with various integrated circuit fabricationtechniques that are known in the art or to be developed.

Memory cells in accordance with the present invention can overcome someof the reliability issues in SONOS and NROM devices. For example, memorycell structures in accordance with the present invention may allow fastFN channel erase methods, while at the same time, maintaining goodcharge retention characteristics. Various embodiments of the memorycells according to the present invention can also alleviate reliance onthe BTBTHH erase method, thereby avoiding device degradation afternumerous P/E cycles.

One example may employ an ultra-thin tunnel dielectric or ultra-thinoxide layer in conjunction with the small hole tunneling barrier heightlayer in embodiments where the tunnel dielectric structure is amultilayer structure. This may provide better stress immunity.Non-volatile memory cells according to the present invention also showlittle degradation after numerous P/E cycles.

Memory cells according to the present invention may employ either ann-channel or a p-channel design, such as shown in FIGS. 1 a and 1 b.FIG. 1 a depicts a cross-sectional view of an n-channel memory cell 100in accordance with one embodiment of the present invention. The memorycell includes a p-type substrate 101 containing at least two n-dopedregions 102 & 104, wherein each of the doped regions 102 & 104 mayfunction as either a source or drain depending upon voltages applied. Asshown in FIG. 1 a, for reference purposes, doped region 102 can serve asthe source and doped region 104 can serve as the drain. The substrate101 further includes a channel region 106 between the two n-dopedregions. Above the channel region 106, on the surface of the substrate101, is a tunnel dielectric structure 120. In certain preferredembodiments, the tunnel dielectric structure 120 can comprise atri-layer thin ONO structure wherein a small hole-tunneling-barrierheight nitride layer 124 is sandwiched between a thin lower oxide layer122 and an upper thin oxide layer 126. The memory cell 100 furtherincludes a charge-trapping (or charge storage) layer 130, preferably anitride, above the tunnel dielectric structure 120, and an insulatinglayer 140, preferably comprising a blocking oxide, disposed above thecharge-trapping layer 130. A gate 150 is disposed on the insulatinglayer 140.

FIG. 1 b, depicts a cross-sectional view of an p-channel memory cell 200in accordance with one embodiment of the present invention. The memorycell includes an n-type substrate 201 containing at least two p-dopedregions 202 & 204, wherein each of the doped regions 202 & 204 mayfunction as either a source or drain. The substrate 201 further includesa channel region 206 between the two p-doped regions. The p-channelmemory cell 200 similarly includes a tunnel dielectric structure 220,comprising a tri-layer thin ONO structure wherein a smallhole-tunneling-barrier height nitride layer 224 is sandwiched between athin lower oxide layer 222 and an upper thin oxide layer 226, acharge-trapping (or charge storage) layer 230, an insulating layer 240,and a gate 250.

Thus, for example, as depicted in FIGS. 1 a and 1 b, memory cells inaccordance with the present invention may include: a multi-layer thinfilm tunnel dielectric structure, including a first silicon oxide layerO1, a first silicon nitride layer N1, and a second silicon oxide layerO2; a charge-storage layer, such as a second silicon nitride layer N2;and an insulating layer such as a third silicon oxide layer O3, on orover (“above”) a substrate, such as a semiconductor substrate (e.g., asilicon substrate). The tunneling dielectric structure allows holetunneling from the substrate to the charge-storage layer during anerase/reset operation of the memory device. Preferably, the tunneldielectric structure in a non-volatile memory cell of the presentinvention has a negligible charge-trapping efficiency, and morepreferably, does not trap charge at all during memory operations.

Charge storage materials such as a silicon nitride layer, HfO₂, andAl₂O₃ may be used as the small hole tunneling barrier height layer in atunnel dielectric structure. In certain preferred embodiments of thepresent invention, an efficient charge storage material, such as asilicon nitride can be used as a charge storage layer in the memorydevice. A blocking oxide that prevents charge loss may serve as aninsulating layer, such as a third silicon oxide layer O3. The memorycells according to the present invention also include a gate or gateelectrode, such as a polysilicon gate, above the insulating layer. Thetunnel dielectric structure, charge storage layer, insulating layer andgate can be formed above the substrate above at least a portion of achannel region, which is defined by and is disposed between a sourceregion and a drain region.

Memory cells according to vaqrious embodiments of the present inventioncomprise a tunnel dielectric structure which can provide fast FN erasespeeds of around 10 msec under a negative gate voltage (Vg), such as aVg of about −10 to about −20 V. On the other hand, the charge retentioncan still be maintained, and, in some examples, may be better than manyconventional SONOS devices. Memory cells according to the presentinvention can also avoid the use of band-to-band hot hole eraseoperations, which, are commonly used in NROM devices. Avoidance of suchband-to-band hot hole erase operations may greatly eliminate hot-holeintroduced damages and such avoidance is therefore desirable.

Referring to FIG. 2, experimental measurements of threshold voltage fora tunnel dielectric structure in accordance with one embodiment of thepresent invention shows that an ultra-thin O1/N1/O2 structure can have anegligible trapping efficiency, as evidenced by the unchanged thresholdvoltage level under successive programming pulses. In the example testedfor FIG. 2, the O1/N1/O2 layers had thicknesses of 30, 30 and 35angstroms (Å), respectively. As shown in FIG. 2, the threshold voltageVt remains steady at approximately 1.9 volts over the course of severalprogram shots using various methods of programming, namely −FNprogramming, +FN programming and CHE (channel hot electron) programming.Thus, such an ultra thin O1/N1/O2 film may serve as a modulated tunneldielectric structure. The results under various charge injection methodsincluding CHE, +FN and −FN all suggest negligible charge trapping.Manufacturing processes or device structures may be designed to minimizeinterfacial traps, so that neither O1/N1 nor N1/O2 interface is active.

FIG. 3 illustrates the erase characteristics of a memory cell having aSONONOS design in accordance with one embodiment of the presentinvention. The memory cell in the embodiment decribed in FIG. 3comprises an n-MOSFET design with an ONO tunnel dielectric strucutrehaving thicknesses of 15 Å, 20 Å and 18 Å, respectively. The memory cellof this embodiment comprises a silicon nitride charge storage layerhaving a thickness of about 70 Å, an insulating silicon oxide layer witha thickness of about 90 Å, and a gate comprising any suitable conductivematerial, for example, n-doped polycrystalline silicon. Referring toFIG. 3, fast FN erase may be achieved, such as within 10 msec, and anexcellent self-convergent erase properties may also be obtained.

FIG. 4 illustrates the charge retention characteristics of a SONONOSdevice in accordance with an embodiment of a memory cell according tothe present invention as described with reference to FIG. 3. As shown,the retention characteristics can be better than those of conventionalSONOS devices, and in terms of magnitude, may be many orders better.

FIGS. 5 a and 5 b are band diagrams which illustrate possible effects ofusing a tunnel dielectric strucutre containing at least one layer havinga small hole-tunneling-barrier height. The band diagram of the tunneldielectric structure, an O1/N1/O2 trilayer in this example, under a lowelectrical field, which may exist during memory data retention, is shownin FIG. 5 a. Direct tunneling as represented by the dotted arrow may beeliminated under low electrical fields, thereby providing good chargeretention during retention states. On the other hand, band diagramoffset under a high electrical field, as shown in FIG. 5 b, can reducethe barrier effect of N1 and O2 such that the direct tunneling throughO1 may occur. A tunnel dielectric strucutre having at least one smallhole-tunneling-barrier height layer may allow efficient FN eraseoperation.

FIGS. 5 c and 5 d illustrate another set of band diagrams in oneexample. For a better band offset condition in one example, thethickness of N1 may be larger than that of O1. The band diagram ofvalence band is plotted at the same electrical field E01=14 MV/cm. Thetunneling probability according to WKB approximation is correlated tothe shadow area. In this example, for N1=O1 in thickness, the bandoffset does not completely screen out the barrier of O2. On the otherhand, for N1>01, the band offset can more easily screen out O1.Therefore, for N1>O1 in thickness, the hole tunneling current may belarger under the same electrical field in O1.

An experiment with measured and simulated hole tunneling currents, asshown in FIG. 6, further describes hole tunneling through a tunneldielectric strucutre according to certain embodiments of the presentinvention. For example, hole tunneling current through the O1/N1/O2dielectric may fall between that of an ultra-thin oxide and a thickoxide. In one example, under a high electrical field, the hole tunnelingcurrent may approximate that of an ultra-thin oxide. However, under alow electrical field, the direct tunneling can be suppressed. As shownin FIG. 6, hole tunneling current is detected through a thin oxide layereven at low electrical field strengths of only 1 MV/cm. Hole tunnelingcurrent is negligible through a thick oxide even at relatively highfield strengths such as, for example, 11–13 mV/cm. However, holetunneling current through an ONO tunnel dielectric structure approachesthat of a thin oxide layer when high electric field strengths arepresent. In FIG. 6, the large current leakage due to hole tunnelingthrough an ultra-thin oxide at low electrical fields can be seen at areaA of the graph. In FIG. 6, hole tunneling current through an O1/N1/O2tunnel dielectric structure at high electric field strengths can be seenat area B of the graph. In FIG. 6, the virtually non-existent tunnelingcurrent through an O1/N1/O2 tunnel dielectric structure and a thickoxide at low electrical fields can be seen at area C of the graph.

Memory cell designs in accordance with the present invention may beapplied to various memory types, including but not limited to, NORand/or NAND-type flash memories.

As noted above, a tunnel dielectric layer may include two or morelayers, including one layer that may provide a smallhole-tunneling-barrier height. In one example, the layer providing asmall hole-tunneling-barrier height may contain silicon nitride. Thelayer may be sandwiched between two silicon oxide layers, therebyforming an O/N/O tunnel dielectric if silicon nitride is used as theintermediate layer. In certain preferred embodiments of the presentinvention, each layer in a tunnel dielectric structure is up to about 4nm thick. In some preferred embodiments, each of the layers in thetunnel dielectric structure can have a thickness of about 1 nm to 3 nm.In one exemplary device, a tri-layer structure may have a bottom layer,such as a silicon oxide layer, of about 10 Å to 30 Å, an intermediatelayer, such as a silicon nitride layer, of about 10 Å to 30 Å, and a toplayer, such as another silicon oxide layer, of about 10 Å to 30 Å. Inone particular example, an O/N/O tri-layer structure having a 15 Åbottom silicon oxide layer, a 20 Å intermediate silicon nitride layer,and an 18 Å top silicon oxide layer may be used.

In one example, a thin O/N/O tri-layer structure shows negligible chargetrapping. Theoretical band diagram and tunneling current analysis, suchas described with reference to FIGS. 5 a, 5 b and 6, may suggest that atunnel dielectric structure, such as an O1/N1/O2 structure havingthicknesses of 3 nm or less for each of the layers, can suppress thehole direct-tunneling at low electric field during retention. At thesame time, it still may allow efficient hole tunneling at high electricfield. This may be because the band offset can effectively screen outthe tunneling barrier of N1 and O2. Therefore, this proposed device mayoffer fast hole tunneling erase, while it is immune from the retentionproblem of the conventional SONOS devices. Experimental analysis showsexcellent endurance and retention properties of memory cells inaccordance with various embodiments of the present invention.

In certain preferred embodiments, the tunnel dielectric structureincludes at least a middle layer and two adjacent layers on opposingsides of the middle layer, wherein each of the middle layer and twoadjacent layers comprises a first material and a second material,wherein the second material has a valence band energy level greater thanthe valence band energy level of the first material and the secondmaterial has a conduction band energy level less than the conductionband energy level of the first material; and wherein the concentrationof the second material is higher in the middle layer than in the twoadjacent layers and the concentration of the first material is higher inthe two adjacent layers than in the middle layer. Preferably, in atunnel dielectric structure in accordance with this embodiment of thepresent invention, the first material comprises oxygen and/or anoxygen-containing compound and the second material comprises nitrogenand/or a nitrogen-containing compound. For example, the first materialcan comprise an oxide, such as silicon oxide, and the second materialcan comprise a nitride, such as Si₃N₄ or Si_(x)O_(y)N_(z).

Tunnel dielectrics in accordance with this aspect of the invention maybe comprised of three or more layers, all of which can contain similarelements (such as Si, N and O), so long as the concentration of thematerial having the smallest hole tunneling barrier height is higherwithin the middle layer than in the two adjacent layers.

In certain tunnel dielectric structures according to the precedingembodiment of the present invention, the second material can be presentin the middle layer in a gradient concentration such that theconcentration of the second material in the middle layer increases fromone adjacent layer/middle layer interface to a maximum concentration ata depth point within the middle layer, and decreases from the maximumconcentration depth point to a lower concentration at the other adjacentlayer/middle layer interface. The increase and decrease in concentrationis preferably gradual.

In still other embodiments of the present invention, the tunneldielectric structure includes at least a middle layer and two adjacentlayers on opposing sides of the middle layer, wherein the two adjacentlayers comprise a first material and the middle layer comprises a secondmaterial, wherein the second material has a valence band energy levelgreater than the valence band energy level of the first material and thesecond material has a conduction band energy level less than theconduction band energy level of the first material; and wherein thesecond material is present in the middle layer in a gradientconcentration such that the concentration of the second material in themiddle layer increases from one adjacent layer/middle layer interface toa maximum concentration at a depth point within the middle layer, anddecreases from the maximum concentration depth point to a lowerconcentration at the other adjacent layer/middle layer interface. Theincrease and decrease in concentration is preferably gradual.Preferably, in a tunnel dielectric structure in accordance with thisembodiment of the present invention, the first material comprises oxygenand/or an oxygen-containing compound and the second material comprisesnitrogen and/or a nitrogen-containing compound. For example, the firstmaterial can comprise an oxide, such as silicon oxide, and the secondmaterial can comprise a nitride, such as Si₃N₄ or Si_(x)O_(y)N_(z).

For example, in embodiments of the present invention where the tunneldielectric layer comprises a tri-layer ONO structure, the bottom oxideand top oxide layers can comprise silicon dioxide and the middle nitridelayer can be comprised of, for example, silicon oxynitride and siliconnitride wherein the concentration of silicon nitride (i.e., the materialhaving the smaller hole tunneling barrier height of the two) is notconstant within the layer, but rather reaches a maximum at some depthpoint within the layer between the two interfaces with the sandwichingoxide layers.

The precise point within the middle layer where the material with thesmallest hole tunneling barrier height reaches its maximum concentrationis not critical, so long as it is present in a gradient and reaches itsmaximum concentration in the tunnel dielectric layer at some pointwithin the middle layer.

The gradient concentration of the material having the smallest holetunneling barrier height can be advantageous in improving variousproperties of non-volatile memory devices, particularly those having aSONONOS, or SONONOS-like structure. For example, retention state chargeloss can be diminished, hole tunneling under high electric fields can beimproved and, to the extent it may occur, charge-trapping in the tunneldielectric can be avoided.

The band diagram of a tunnel dielectric layer can be advantageouslymodified in accordance with this aspect of the present invention suchthat the valence band energy level and the conduction band energy levelof the middle layer do not have a constant value, but rather vary acrossthe thickness of the layer with the concentration of the material havingthe smallest hole tunneling barrier height. Referring to FIG. 5 e,modification of an ONO tri-layer tunnel dielectric in accordance withthis aspect of the invention is shown via a band diagram. The middlelayer (Layer-2) is comprised of silicon nitride. The outer layers(Layer-1 and Layer-3) are comprised of silicon dioxide. Theconcentration of silicon nitride in Layer-2 is varied such that thevalence band energy level and the conduction band energy level reach amaximum and minimum value, respectively, at the depth in Layer-2 wherethe concentration of silicon nitride is highest. Three possible siliconnitride concentration gradients are shown in FIG. 5 e, depicted bydashed lines representing the variable valence band energy conductionband energy levels that result from the concentration gradients. Asshown in FIG. 5 e, by the circles on the dashed lines representing threealternative silicon nitride concentration maximums within Layer-2, thelowest valence band energy level and the highest conduction band energylevel coincide with the silicon nitride concentration maximum.

Multi-layer tunnel dielectric structures in accordance with suchembodiments of the present invention, can be prepared in a variety ofways. For example, a first silicon dioxide or silicon oxynitride layercan be formed using any number of conventional oxidation approachesincluding, but not limited to thermal oxidation, radical (ISSG)oxidation, and plasma oxidation/nitridation, as well as chemical vapordeposition processes. A middle layer with a gradient concentration ofSiN can then be formed, for example, via chemical vapor depositionprocesses, or alternatively, by plasma nitridation of excess oxide oroxynitride formed on top of the first layer. A third layer, the upperoxide layer, can then be formed, for example, by oxidation or chemicalvapor deposition.

A charge storage layer can then be formed over the tunnel dielectricstructure. In one example, a charge storage layer of about 5 nm to 10 nmmay be formed over the tunnel dielectric structure. In one particularexample, a silicon nitride layer of about 7 nm or thicker may be used.The insulating layer above the charge storage layer may be about 5 nm to12 nm. For example, a silicon oxide layer of about 9 nm or thicker maybe used. And the silicon oxide layer may be formed by a thermal processconverting at least a portion of a nitride layer to form the siliconoxide layer. Any method, known or to be developed, for forming layers ofsuitable materials described herein can be used to deposit or formtunnel dielectric layers, charge-storage layers and/or insulatinglayers. Suitable methods include, for example, thermal growth methodsand chemical vapor deposition methods.

In one example, a thermal conversion process may provide a high densityor concentration of interfacial traps that can enhance the trappingefficiency of a memory device. For example, thermal conversion ofnitride can be carried out at 1000° C., while the gate flow ratio isH2:O2=1000:4000 sccm.

In addition, because silicon nitride generally has very low (about 1.9eV) hole barrier, it may become transparent to hole tunneling under highfield. Meanwhile, the total thickness of a tunnel dielectric, such as anONO structure, may prevent direct tunneling of electrons under a lowelectric field. In one example, this asymmetrical behavior may provide amemory device offering not only fast hole-tunneling erase, but alsoreduction or elimination of charge leakage during retention.

An exemplary device may be fabricated by 0.12 μm NROM/NBit technologies.Table 1 shows the device structure and parameters in one example. Theproposed tunnel dielectric with an ultra-thin O/N/O may alter the holetunneling current. A thicker (7 nm) N2 layer may serve as acharge-trapping layer and an O3 (9 nm) layer may serve as the blockinglayer in one example. Both N2 and O3 may be fabricated using NROM/NBittechnologies.

TABLE 1 Layer Approximate Thickness (Angstroms) Bottom Oxide (O1) 15Inter Nitride (N1) 20 Inter Oxide (O2) 18 Trapping Nitride (N2) 70Blocking Oxide (O3) 90 Gate: N+ - polysilicon Channel length: 0.22 μmChannel width: 0.16 μm

In certain embodiments of the present invention, a gate can comprise amaterial having a work function greater than that of N⁺ polysilicon. Incertain preferred embodiments of the present invention, such a high workfunction gate material can comprise a metal such as, for example,platinum, iridium, tungsten, and other noble metals. Preferably, thegate material in such embodiments has a work function greater than orequal to about 4.5 eV. In particularly preferred embodiments, the gatematerial comprises a high work function metal such as, for example,platinum or iridium. Additionally, preferred high work functionmaterials include, but are not limited to P⁺ polysilicon, and metalnitrides such as, for example, titanium nitride and tantalum nitride. Inparticularly preferred embodiments of the present invention, the gatematerial comprises platinum.

An exemplary device in accordance with an embodiment of the presentinvention having a high work function gate material may also befabricated by 0.12 μm NROM/NBit technologies. Table 2 shows the devicestructure and parameters in one example. The proposed tunnel dielectricwith an ultra-thin O/N/O may alter the hole tunneling current. A thicker(7 nm) N2 layer may serve as a charge-trapping layer and an O3 (9 nm)layer may serve as the blocking layer in one example. Both N2 and O3 maybe fabricated using NROM/NBit technologies.

TABLE 2 Layer Approximate Thickness (Angstroms) Bottom Oxide 15 InterNitride 20 Inter Oxide 18 Trapping Nitride (N2) 70 Blocking Oxide 90Gate: Platinum Channel length: 0.22 μm Channel width: 0.16 μm

Memory cells in accordance with high work function gate materialembodiments of the present invention exhibit erase properties which areeven more improved over other embodiments. High work function gatematerials suppress gate electron injection into the trapping layer. Incertain embodiments of the present invention wherein the memory cellscomprise an N⁺ polysilicon gate, hole tunneling into the charge-trappinglayer during erase occurs simultaneously with gate electron injection.This self-converging erase effect results in higher threshold voltagelevels in the erased state, which can be undesirable in NANDapplications. Memory cells in accordance with high work function gatematerial embodiments of the present invention can be used in varioustype of memory applications including, for example, NOR- and NAND-typememories. However, the memory cells according to high work function gatematerial embodiments of the present invention are particularly suitablefor use in NAND applications where elevated threshold voltages in theerased/reset state can be undesirable. Memory cells in accordance withhigh work function gate material embodiments of the present inventioncan be erased via hole tunneling methods and preferably via −FN erasingoperations.

An exemplary device having an ONO tunneling dielectric and an N⁺polysilicon gate may be programmed by conventional SONOS or NROM methodand erased by channel FN hole tunneling. FIG. 7 a shows the erasecharacteristics of an exemplary SONONOS device having an ONO tunnelingdielectric in one example. Referring to FIG. 7 a, a higher gate voltageresults in a faster erase speed. It also has a higher saturation Vt,because gate injection is also stronger and the resulting dynamicbalance point (which determines the Vt) is higher. This is shown on theright-hand side of the graph as the threshold voltage reaches a minimumat values of from about 3 to about 5 volts depending upon the erase gatevoltage. The hole tunneling current can be extracted by a transientanalysis method by differentiating the curves in FIG. 7 a. The extractedhole current from the measurement in FIG. 7 a is illustrated in FIG. 6as discussed above. For comparison, there is also plotted simulated holetunneling current using WKB approximation. The experimental results arein reasonable agreement with our prediction. The tunneling currentthrough the O1/N1/O2 stack approaches that of the ultra-thin O1 under ahigh electric field, while it is turned-off under a low electric field.

In accordance with certain embodiments of memory cells of the presentinvention having high work function gate materials, wherein the highwork function gate suppresses gate electron injection, the thresholdvoltage of the device in an erased or reset state can be much lower, andeven negative, depending upon erase time. The threshold voltage valuesof a memory device in accordance with one embodiment of the presentinvention wherein the gate is comprised of platinum and the tunneldielectric layer comprises a 15/20/18 angstrom ONO structure are shownin FIG. 7 b. As shown in FIG. 7 b, at a similar gate voltage (−18 V)during a −FN erase operation, the threshold voltage of the device can beset below −3V. The corresponding capacitance versus gate voltage valuesfor the device are shown in FIG. 7 c.

Moreover, retention properties of memory devices in accordance with highwork function gate material embodiments the present invention areimproved. The retention properties of a memory device having a platinumgate are shown in FIG. 7 d wherein the capacitance is graphed as afunction of gate voltage following erase and program, and then 30minutes after each operation and two hours after each operation. Minimaldeviation is observed.

Memory cells in accordance with various embodiments of the presentinvention may be operated with at least two separate schemes. Forexample, CHE programming with reverse read (mode 1) may be used toperform a 2-bits/cell operation. Additionally, low-power +FN programming(mode 2) may also be used for a 2-bits/cell operation. Both modes canuse the same hole tunneling erase method. Mode 1 may preferably be usedfor a virtual ground array architecture for NOR-type flash memories.Mode 2 may preferably be used for NAND-type flash memories.

As an example, FIG. 8 shows the excellent endurance properties of avirtual ground array architecture NOR-type flash memory in accordancewith one embodiment of the present invention under mode 1 operation.Erase degradation of such memory devices having a tunnel dielectricstrucutre does not occur, because hole tunneling erase (Vg=−15 V) is auniform channel erase method. The corresponding IV curves are also shownin FIG. 9, which suggest little degradation of the device after numerousP/E cycles. In one example, this may be because ultra-thin oxide/nitridelayers possess good stress immunity properties. Additionally, the memorydevice is free of hot-hole introduced damages. The endurance propertiesof a NAND-type flash memory in accordance with one embodiment of thepresent invention under Mode 2 operation are shown in FIG. 10. For afaster convergent erasing time, one may use a larger bias (Vg=−16 V).Excellent endurance may also be obtained in this example.

The charge retention of an exemplary SONONOS device in accordance withone embodiment of the present invention is shown in FIG. 4, where only a60 mV charge loss is observed after 100 hours. The improvement ofretention is many orders of magnitude better than conventional SONOSdevices. VG-accelerated retention test also shows that direct tunnelingcan be suppressed at the low electrical field. FIG. 11 illustrates anexample of a VG-accelerated retention test for a 10K P/E cycled device.The charge loss is small at −VG stress after a 1000 sec stress,indicating that the hole direct tunneling at small electrical field canbe suppressed.

Accordingly, the SONONOS design identified in the above examples mayprovide a fast hole tunneling erase with excellent endurance properties.As noted above, the design may be implemented in both NOR and NAND-typenitride-storage flash memories. Additionally, a memory array inaccordance with the present invention may include multiple memorydevices with similar or different configurations.

In various embodiments of arrays according to the present invention,memory cells according to the present invention may be used in place ofconventional NROM or SONOS devices in a virtual ground arrayarchitecture. The reliability problems and erase degradations may besolved or mitigated by using FN hole tunneling instead of hot-holeinjection. Without limiting the scope of the invention to the specificstructures described below, various operation methods in accordance withmemory arrays of the present invention are described below for exemplaryNOR virtual ground array architectures.

CHE or CHISEL (channel initiated secondary electron) programming andreverse read may be used for 2-bit/cell memory array. And the erasemethod may be a uniform channel FN hole tunneling erase. In one example,the array architecture may be a virtual ground array or a JTOX array.With reference to FIGS. 12 a–20, an O1/N1/O2 tri-layer structure may beused as the tunnel dielectric, each layer having about 3 nm or less inthickness to provide hole direct tunneling. With reference to FIGS. 12a–20, N2 may be thicker than 5 nm to provide a high trapping efficiency.An insulating layer, O3, may be a silicon oxide layer formed by wetoxidation, such as a wet converted top oxide (silicon oxide), to providea large density of traps at the interface between O3 and N2. O3 may beabout 6 nm or thicker to prevent charge loss from this silicon oxidelayer.

FIGS. 12 a and 12 b illustrate an example of a virtual ground arrayarchitecture incorporating the memory cells discussed above, such asmemory cells having a tri-layer ONO tunnel dielectric. In particular,FIG. 12 a illustrates an equivalent circuit of a portion of a memoryarray, and FIG. 12 b illustrates an exemplary layout of a portion of thememory array.

In addition, FIG. 13 illustrates a schematic diagram of thecross-sectional view of several memory cells incorporated in the array.In one example, the buried diffusion (BD) areas may be N+-dopedjunctions for the source or drain regions of the memory cells. Thesubstrate may be a p-type substrate. In order to avoid possiblebreakdown of the BDOX areas (oxide above BD) during −FN erase, a thickBDOX (>50 nm) may be used in one example.

FIGS. 14 a and 14 b illustrate possible electrical RESET schemes for anexemplary virtual ground array incorporating 2 bits/cell memory cellshaving a tunnel dielectric design discussed above. Before performingfurther P/E cycles, all the devices may first undergo an electrical“RESET”. A RESET process may ensure the Vt uniformity of memory cells inthe same array and raise the device Vt to the convergent erased state.For example, applying Vg=−15 V for 1 sec, as shown in FIG. 14 a, mayhave the effect of injecting some charge into a charge trapping layer ofsilicon nitride to reach a dynamic balancing condition. With the RESET,even memory cells that are non-uniformly charged due, for example, tothe plasma charging effect during their fabrication processes may havetheir Vt converged. An alternative way for creating a self-convergingbias condition is to provide bias for both gate and substrate voltages.For example, referring to FIG. 14 b, Vg=−8 V and P-well=+7 V may beapplied.

FIGS. 15 a and 15 b illustrate programming schemes for an exemplaryvirtual ground array incorporating 2 bits/cell memory cells having atunnel dielectric design discussed above. Channel hot-electron (CHE)programming may be used to program the device. For Bit-1 programmingillustrated in FIG. 15 a, the electrons are locally injected into thejunction edge above BLN (bit line N). For Bit-2 programming shown inFIG. 15 b, the electrons are stored above BLN-I. Typical programmingvoltage for WL (word line) is around 6 V to 12 V. Typical programmingvoltage for BL (bit line) is about 3 to 7 V, and the p-well may be keptgrounded.

FIGS. 16 a and 16 b illustrate reading schemes for an exemplary virtualground array incorporating 2 bits/cell memory cells having a tunneldielectric design discussed above. In one example, reverse read is usedto read the device to perform a 2 bits/cell operation. Referring to FIG.16 a, for reading Bit-1, BLN-1 is applied with a suitable read voltage,such as 1.6 V. Referring to FIG. 16 b, for reading bit-2, BLN is appliedwith a suitable read voltage, such as 1.6V. In one example, the readingvoltage may be in the range of about 1 to 2 V. The word lines and theP-well may be 13 kept grounded. However, other modified read schemes,such as a raised-Vs reverse read method can be performed. For example, araised-Vs reverse read method may use Vd/Vs=1.8/0.2 V for reading Bit-2,and Vd/Vs=0.2/1.8 for reading Bit-1.

FIGS. 14 a and 14 b also illustrate sector erase schemes for anexemplary virtual ground array incorporating 2 bits/cell memory cellshaving a tunnel dielectric design discussed above. In one example,sector erase with channel hole tunneling erase may applied to erase thememory cells simultaneously. An ONO tunnel dielectric in a memory cellhaving the SONONOS structure may offer a fast erase, which may occur inabout 10 to 50 msec and a self-convergent channel erase speed. In oneexample, a sector erase operation condition may be similar to a RESETprocess. For example, referring to FIG. 14 a, applying VG= about −15 Vat the WL's simultaneously and leaving all the BL's floating may achievea sector erase. And the p-well may be kept grounded.

Alternatively, referring to FIG. 14 b, applying about −8 V to the WL'sand about +7 V to the p-well may also achieve a sector erase. In someexamples, a complete sector erase operation may be carried out within100 msec or less without having any over-erase or hard-to-erase cells.The device design discussed above may facilitate a channel eraseproviding excellent self-converging properties.

FIG. 17 illustrates the erase characteristics in one example of using anSONONOS device. An example of an SONONOS device may have the thicknessof O1/N1/O2/N2/O3 respectively as about 15/20/18/70/90 Angstroms, withan N+-polysilicon gate and thermally converted top oxide as O3. Theerase speeds for various gate voltages are shown. A higher gate voltageresults in a faster erase speed.

However, the convergent Vt is also higher. This is because gateinjection is more active under higher gate voltages. To reduce gateinjection, P+-polysilicon gate or other metal gate with a high workfunction may be used alternatively as the gate material to reduce thegate-injected electrons during the erase.

FIG. 18 illustrates the endurance properties of using SONONOS devices ina virtual ground array architecture. The endurance properties of in someexamples are excellent. The programming condition is Vg/Vd=8.5/4.4 V,0.1 μsec for Bit-1 and Vg/Vs=8.5/4.6 V, 0.1 μsec for Bit-2. The FN erasemay use Vg=−15 V for about 50 msec to erase the two bits simultaneously.Because the FN erase is self-convergent uniform channel erase,hard-to-erase or over-erase cells usually do not present. In someexamples, the devices proposed above show excellent endurance propertieseven without using a Program/Erase verifying or stepping algorithim.

FIGS. 19 a and 19 b illustrate I-V characteristics during P/E cycles inone example. The corresponding I–V curves in both log scale (FIG. 19 a)and linear scale (FIG. 19 b) are shown. In one example, an SONONOSdevice possesses little degradations after numerous P/E cycles, suchthat both the sub-threshold swing (S.S.) and trans-conductance (gm) arealmost the same after numerous cycles. This SONONOS device possessessuperior endurance properties than NROM device. One reason may be thathot-hole injection is not used. Additionally, an ultra-thin oxide asnoted above may possess better stress immunity properties than a thicktunnel oxide.

FIG. 20 illustrates a CHISEL programming scheme in one example. Analternative way to program the device is to use CHISEL programmingscheme, which uses negative substrate bias enhanced impact ionization toincrease the hot carrier efficiency. The programming current can be alsoreduced due to the body effect. Typical condition is illustrated in thisfigure, where substrate is applied with a negative voltage (−2 V), andthe junction voltage is reduced to about 3.5 V. For conventional NROMdevices and technologies, CHISEL programming is not applicable becauseit may inject more electrons near the channel center region. Andhot-hole erase is inefficient to remove the electrons near the channelcenter region in the conventional NROM devices.

FIGS. 21 a and 21 b illustrate the design of a JTOX virtual ground arrayin one example. The JTOX virtual ground array provides an alternativeimplementation of using SONONOS memory cells in a memory array. In oneexample, one difference between the JTOX structure and a virtual groundarray is that the devices in the JTOX structure that are isolated by STIprocesses. A typical layout example is illustrated in FIG. 21 a. FIG. 21b illustrates a corresponding equivalent circuit, which is the same asthat of a virtual ground array.

As noted above, memory cell structures in accordance with the presentinvention are suitable for both NOR- and NAND-type flash memories. Thefollowing will describe additional examples of memory array designs andtheir operation methods. Without limiting the scope of the invention tothe specific structures described below, various operation methods inaccordance with memory arrays of the present invention are describedbelow for exemplary NAND architectures.

As noted above, n-channel SONONOS memory devices having an ONO tunnelingdielectric may be used in a memory device. FIGS. 22 a and 22 billustrate an example of a NAND array architecture. FIGS. 23 a and 23 billustrate the cross-sectional views of an exemplary memory array designfrom two different directions. In some examples, the operation methodsof a memory array may include +FN programming, self-convergingreset/erase, and reading methods. Additionally, circuit operationmethods may be included to avoid program disturb in some examples.

In addition to the single-block gate structure design, a split-gatearray, such as a NAND array using SONONOS devices positioned between twotransistor gates which are located next to the source/drain regions, mayalso be used. In some examples, a split-gate design may scale downdevice dimension to F=30 nm or below. Furthermore, the devices may bedesigned to obtain good reliability, to reduce or eliminate theinter-floating-gate coupling effect, or to achieve both. As discussedabove, an SONONOS memory device may provide excellent self-convergingerase, which may help sector-erase operations and Vt distributioncontrol. Furthermore, a tightened erased state distribution mayfacilitate multi-level applications (MLC).

By using certain designs for a memory array structure, the effectivechannel length (Leff) may be enlarged to reduce or eliminateshort-channel effects. Some examples may be designed to use no diffusionjunctions, thereby avoiding the challenges in providing shallowjunctions or using pocket implantations during the manufacturingprocesses of memory devices.

FIG. 1 illustrates an example of a memory device having an SONONOSdesign. In addition, Table 1 noted above illustrates an example ofmaterials used for different layers and their thicknesses. In someexamples, P+-polysilicon gate may be used to provide a lower saturatedReset/Erase Vt, which may be achieved by reducing gate injection.

FIGS. 22 a and 22 b illustrate an example of a memory array, such as anSONONOS-NAND array having memory cells in accordance with embodimentdescribed in Table 1, with diffusion junctions. In one example, separatedevices may be isolated from each other by various isolation techniques,such as by using shallow-trench isolation (STI) or the isolationtechnique of silicon-on-insulator (SOI). Referring to FIG. 22 a, amemory array may include multiple bit lines, such as BL1 and BL2, andmultiple word lines, such as WL1, WLN-1, and WLN. Additionally, thearray may include source line transistor(s) (or source-line-selectingtransistor(s) or SLTs) and bit line transistor(s) (or bit-line-selectingtransistor(s) or BLTs). As illustrated, the memory cells in the arraymay use an SONONOS design, and the SLT and BLT may include n-typemetal-oxide-semiconductor field-effect transistors (NMOSFETs).

FIG. 22 b illustrates an exemplary layout of a memory array, such as aNAND array. Referring to FIG. 22 b, Lg is the channel length of memorycells, and Ls is the space between each separate lines of memorydevices. Additionally, W is the channel width of memory cells, and Ws isthe width of isolation areas between separate bit lines or source/drainareas, which may be the STI width in one example.

Referring again to FIGS. 22 a and 22 b, the memory devices may beconnected in series and form a NAND array. For example, a string ofmemory devices may include 16 or 32 memory devices, providing a stringnumber of 16 or 32. The BLTs and SLTs may be used as selectingtransistors to control the corresponding NAND strings. In one example,the gate dielectric for BLTs and SLTs may be a silicon oxide layer thatdoes not include a silicon nitride trapping layer. Such configuration,although not necessarily required in every case, may avoid possible Vtshift of BLTs and SLTs during the operations of the memory array in someexamples. Alternatively, the BLTs and SLTs may use the combination ofONONO layers as their gate dielectric layers.

In some examples, the gate voltages applied to BLTs and SLTs may be lessthan 10 V, which may cause less gate disturb. In cases where the gatedielectric layer of BLTs and SLTs may be charged or trapped withcharges, additional −Vg erase can be applied to the gates of BLT or SLTto discharge their gate dielectric layers.

Referring again to FIG. 22 a, each BLT may be coupled to a bit-line(BL). In one example, a BL may be a metal line having the same orapproximately the same pitch to that of STI. Also, each SLT is connectedto a source line (SL). The source line is parallel to the WL andconnected to the sense amplifier for read sensing. The source line maybe a metal, such as tungsten, or polysilicon line, or a diffusionN+-doped line.

FIG. 23 a illustrates a cross-sectional view of an exemplary memoryarray, such as an SONONOS-NAND memory array, along the channel-lengthdirection. Typically, Lg and Ls is approximately equal to F, whichgenerally represents the critical dimension of a device (or node). Thecritical dimension may vary with the technologies used for fabrication.For example, F=50 nm stands for using a 50 nm node. FIG. 23 billustrates a cross-sectional view of an exemplary memory array, such asan SONONOS-NAND memory array, along the channel-width direction.Referring to FIG. 23 b, the pitch in the channel-width direction isapproximately equal or slightly larger than that in the channel lengthdirection. Therefore, the size of a memory cell is approximately4F2/cell.

In examples of manufacturing a memory array, such as the arrays notedabove, the processes may involve using only two primary masks orlithography processes, such as one for the polysilicon (word line) andanother for STI (bit lines). In contrast, the manufacturing of NAND-typefloating gate devices may require at least two-poly processing andanother inter-poly ONO processing. Accordingly, the structure andmanufacturing processes of the proposed devices may be simpler thanthose of NAND-type floating gate memories.

Referring to FIG. 23 a, in one example, the spaces (Ls) between wordlines (WLs) may be formed with shallow junctions, such as shallowjunctions of N+-doped regions, which may serve as source or drainregions of the memory devices. As illustrated in FIG. 23A, additionalimplantation and/or diffusion process, such as a tilt-angle pocketimplantation, may be carried out to provide one or more “pocket” regionsor pocket extensions of junctions that neighbor one or more of theshallow junction regions. In some examples, such configuration mayprovide better device characteristics.

In examples where STI is used of isolating separate memory devices, thetrench depth of STI regions may be larger than the depletion width inp-well, especially when the junction bias used is raised higher. Forexample, the junction bias may be as high as about 7V for programinhibited bit line(s) (unselected bit line(s) during programming). Inone example, the depth of STI regions may be in the range of about 200to 400 nm.

After a memory array is manufactured, a reset operation may be performedto tighten the Vt distribution first before other operations of thememory array. FIG. 24 a illustrates an examples of such operation. Inone example, before other operations start, one may first apply VG=about −7 V and VP-well=+8 V to reset the array (The voltage drop of VGand VP-Well can be partitioned into the gate voltage into each WL andp-well).During RESET, the BL's can be floating, or raised to the samevoltage as the P-Well. As illustrated in FIG. 24 b, the reset operationmay provide excellent self-convergent properties. In one example, evenSONONOS devices are initially charged to various Vts, the resetoperation can “tighten” them to a Reset/Erase state. In one example, thereset time is about 100 msec. In that example, the memory array may usen-channel SONONOS devices with ONONO=15/20/18/70/90 angstroms having anN+-polysilicon gate with Lg/W=0.22/0.16 um.

Generally, traditionally floating-gate devices are not capable ofproviding self-converging erase. In contrast, SONONOS devices may beoperated with converging Reset/Erase methods. In some examples, thisoperation may become essential because the initial Vt distribution isoften in a wide range due to certain process issues, such as processnon-uniformity or plasma charging effects. The exemplary self-converging“Reset” may help to tighten, or narrow the range of, the initial Vtdistribution of memory devices.

In one example of programming operations, the selected WL may be appliedwith a high voltage, such as a voltage of about +16 V to +20 V, toinduce channel +FN injection. Other PASS gates (other unselected WL's)may be turned on to induce the inversion layer in a NAND string. +FNprogramming may be a low-power method in some examples. In one example,parallel programming methods such as page programming with 4K Bytescells in parallel can burst the programming throughput to more than 10MB/sec, while the total current consumption can be controlled within 1mA. In some examples, to avoid program disturb in other BLs, a highvoltage, such as a voltage of about 7 V may be applied to other BLs sothat the inversion layer potential is raised higher to suppress thevoltage drop in the unselected BLs (such as cell B in FIG. 25).

In examples of read operations, the selected WL may be raised to avoltage that is between an erased state level (EV) and a programmedstate level (PV). Other WLs may serve as the “PASS gates” so that theirgate voltages may be raised a voltage higher than PV. In some examples,erase operations may be similar to the reset operation noted above,which may allow self-convergence to the same or similar reset Vt.

FIG. 25 illustrates an example of operating a memory array. Programmingmay include channel +FN injection of electrons into an SONONOS nitridetrapping layer. Some examples may include applying Vg= about +18 V tothe selected WLN-1, and applying VG= about +10 V to other WLs, as wellas the BLT. The SLT can be turned off to avoid channel hot electroninjection in cell B. In this example, because all the transistors in theNAND string are turned-on, the inversion layer passes through thestrings. Furthermore, because BL1 is grounded, the inversion layer inBL1 has zero potential. On the other hand, other BLs are raised to ahigh potential, such as a voltage of about +7 V, so that the inversionlayer of other BLs are higher.

In particular, for cell A, which is the cell selected for programming,the voltage drop is about +18 V, which causes +FN injection. And the Vtmay be raised to PV. For cell B, the voltage drop is +11 V, causing muchless +FN injection, as FN injection is sensitive to Vg. For cell C, only+10 V is applied, causing no or negligible +FN injection. In someexamples, a programming operation is not limited to the techniqueillustrated. In other words, other adequate program inhibit techniquesmay be applied.

FIGS. 24 a, 26, and 27 further illustrate some examples of arrayoperations and illustrate the endurance and retention properties of someexamples. As illustrated, the device degradation after a number ofoperation cycles may remain very small. FIG. 24A illustrates anexemplary erase operation, which may be similar to a reset operation. Inone example, the erase is performed by sector or block. As noted above,the memory devices may have good self-converging erase property. In someexamples, the erase saturation Vt may be dependent on Vg. For example, ahigher Vg may cause a higher saturated Vt. As illustrated in FIG. 26B,the convergent time may be around 10 to 100 msec.

FIG. 27 illustrates an exemplary reading operation. In one example,reading may be performed by applying a gate voltage that is between anerased state Vt (EV) and a programmed state Vt (PV). For example, thegate voltage may be about 5 V. On the other hand, other WLs and BLT andSLT are applied with a higher gate voltage, such as a voltage of about+9 V, to turn-on all the other memory cells. In one example, if Vt ofcell A is higher than 5 V, the read current may be very small (<0.1 uA).If Vt of cell A is lower than 5 V, the read current may be higher (>0.1uA). As a result, the memory state, i.e. the stored information, can beidentified.

In some examples, the pass gate voltage for other WLs should be higherthan the high-Vt state or the programmed state Vt, but not too high totrigger gate disturb. In one example, the PASS voltage is in the rangeof about 7 to 10 V. The applied voltage at the BL may be about 1 V.Although a larger read voltage may induce more current, the read disturbmay become more apparent in some examples. In some examples, the sensingamplifier can be either placed on a source line (source sensing) or on abit line (drain sensing).

Some examples of NAND strings may have 8, 16, or 32 memory devices perstring. A larger NAND string may save more overhead and increase arrayefficiency. However, in some examples, the read current may be smallerand disturb may become more apparent. Therefore, adequate numbers ofNAND string should be chosen based on various design, manufacture, andoperation factors.

FIG. 28 illustrates the cycle endurance of certain exemplary devices.Referring to FIG. 28, P/E cycles with +FN program and −FN erase may becarried out, and the results suggest good endurance characteristics. Inthis example, the erase condition is Vg= about −16 V for 10 msec. Insome examples, only single shot of erase is needed and verification ofstatus is not necessary. The memory Vt window is good withoutdegradation.

FIGS. 29 a and 29 b illustrate the IV characteristics of exemplarymemory devices using different scales. In particular, FIG. 29 aillustrates a small swing degradation of the device, and FIG. 29 billustrates a small gm degradation of the device. FIG. 30 illustratesthe retention characteristics of an exemplary SONONOS device. Referringto FIG. 30, a good retention is provided by having less than 100 mVcharge loss for device operated after 10K cycles and after leaving for200 hours at room temperature. FIG. 30 also illustrates an acceptablecharge loss at high temperatures.

In some examples, a split-gate design, such as a split-gate SONONOS-NANDdesign, may be used to achieve a more aggressive down-scaling of amemory array. FIG. 31 illustrates an example of using such design.Referring to FIG. 31, the spaces (Ls) between each word line, or betweentwo neighboring memory devices sharing the same bit line, may bereduced. In one example, Ls may be shrunk to about or less than 30 nm.As illustrated, the memory devices using a split-gate design along thesame bit line may share only one source region and one drain region. Inother words, a split-gate SONONOS-NAND array may use no diffusionregions or junctions, such as N+-doped regions, for some of the memorydevices. In one example, the design may also reduce or eliminate theneed for shallow junctions and neighboring “pockets”, which in someexamples may involve a more complicated manufacturing process.Furthermore, in some examples, the design is less affected byshort-channel effects, because the channel length has been increased,such as increased to Lg=2F-Ls in one example.

FIG. 32 illustrates an exemplary manufacturing process of a memory arrayusing a split-gate design. The schematic diagram is merely anillustrative example, and the memory array may be designed andmanufactured in various different ways. Referring to FIG. 32, aftermultiple layers of materials for providing the memory devices areformed, those layers may be patterned using a silicon oxide structure asa hard mask formed over those layers. For example, the silicon oxideregions may be defined by lithography and etching processes. In oneexample, the pattern used for defining the initial silicon oxide regionsmay have a width of about F and the space between the silicon oxideregions of about F, resulting a pitch of about 2 F. After the initialsilicon oxide regions are patterned, silicon oxide spacers may then beformed surrounding the patterned regions to enlarge each silicon oxideregion and narrow their spacing.

Referring again to FIG. 32, after the silicon oxide regions are formed,they are used as a hard mask to define or pattern their underlyinglayers to provide one or more memory devices, such as multiple NANDstrings. In addition, insulating materials, such as silicon oxide, maybe used to fill in the spaces, such as Ls spaces shown in FIG. 32,between the neighboring memory devices.

In one example, the space Ls between neighboring memory devices alongthe same bit line may be in the range of about 15 nm to about 30 nm. Asnoted above, the effective channel length may be enlarged to 2F-Ls inthis example. In one example, if F is about 30 nm and Ls is about 15 nm,Leff is about 45 nm. For the operation of those exemplary memorydevices, the gate voltage may be reduced to below 15 V. In addition, theinter-polysilicon voltage drop between word lines may be designed to beno larger than 7V to avoid breakdown of the spacers in the Ls spaces. Inone example, this may be achieved by having an electric field of lessthan 5 MV/cm between neighboring word lines.

The Leff with diffusion junctions for conventional NAND floating-gatedevices is about half of the their gate length. In contrast, if F isabout 50 nm and Leff is about 30 nm, Leff is about 80 nm for theproposed design (the split-gate NAND) in one example. The longer Leffcan provide better device characteristics by reducing or eliminating theimpact of short-channel effects.

As illustrated above, a split-gate NAND design may further shrink thespace (Ls) between neighboring memory cells of the same bit line. Incontrast, traditional NAND-type floating-gate devices may not provide asmall spacing, because inter-floating-gate coupling effect may lose thememory window The inter-floating gate coupling is the interferencebetween adjacent memory cells when the coupling capacitance betweenadjacent floating gate is high (the space between the floating gates issmall so that the coupling capacitance between the adjacent floatinggates becomes very high such that read disturb happens). As noted above,the design may eliminate the need to fabricate certain diffusionjunctions, and the inversion layer can be directly connected if all theword lines are turned on. Therefore, the design may simplify themanufacturing process of memory devices.

As illustrated, some examples noted above, including the structuraldesign, array design, and operation of memory devices, may providedesirable array dimension, good reliability, good performance, or thecombination of any of them. Some examples noted may be applicable fordown-scaling the dimensions of non-volatile flash memories, such as NANDflash memories and flash memory for data applications. Some examples mayprovide SONONOS devices with uniform and self-converging channelhole-tunneling erase. Some examples also may provide good endurance ofmemory devices and reduce certain no hard-to-erase or over-erase issues.Also, good device characteristics, such as small degradations after P/Ecycles and good charge retention, may be provided. Device uniformitywithin a memory array may be provided without having erratic bits orcells. Furthermore, some examples may provide good short-channel devicecharacteristics via a split-gate NAND design, which may offer a bettersense margin during the operations of the memory devices.

The foregoing disclosure of the preferred embodiments of the presentinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. It will be appreciated bythose skilled in the art that changes could be made to the embodimentsdescribed above without departing from the broad inventive conceptthereof. It is understood, therefore, that this invention is not limitedto the particular embodiments disclosed, but it is intended to covermodifications within the spirit and scope of the present invention asdefined by the appended claims.

1. A method of operating a memory array comprising a plurality of memorycells disposed on a semiconductor substrate, wherein each of said memorycells comprises: a source region and a drain region disposed below asurface of the substrate and separated by a channel region; a tunneldielectric structure disposed above the channel region, the tunneldielectric structure comprising at least one layer having a smallhole-tunneling-barrier height; a charge storage layer disposed above thetunnel dielectric structure; an insulating layer disposed above thecharge storage layer; and a gate electrode disposed above the insulatinglayer; the method comprising: applying self-converging reset/erasevoltages to the substrate and the gate electrode in each memory cell tobe reset/erased; programming at least one of the plurality of memorycells; and reading at least one of the plurality of memory cells byapplying a voltage between an erased state level and a programmed statelevel of at least one of the memory devices.
 2. The method according toclaim 1, further comprising applying self-converging reset/erasevoltages to the substrate and the gate electrode in the at least oneprogrammed memory cell.
 3. The method according to claim 1, whereinapplying self-converging reset/erase voltages comprises applying anegative gate voltage, V_(g), and a substrate voltage, V_(s), having apotential difference, V_(g)−V_(s), of from about −20 V to about −12 V.4. The method according to claim 1, wherein applying self-convergingreset/erase voltages to the at least one programmed memory cellcomprises applying a negative gate voltage, V_(g), and a substratevoltage, V_(s), having a potential difference, V_(g)−V_(s), of fromabout −20 V to about −12 V.
 5. The method according to claim 1, whereinapplying self-converging reset/erase voltages comprises applying a gatevoltage of about −20 to about −12 V and grounding the substrate.
 6. Themethod according to claim 1, wherein applying self-convergingreset/erase voltages comprises applying a gate voltage of about −10 toabout −2 V and a substrate voltage of about +5 V to about +10 V.
 7. Themethod according to claim 1, wherein the memory array has a NORarchitecture, and wherein programming comprises channel hot electroninjection.
 8. The method according to claim 1, wherein the memory arrayhas a NAND architecture, and wherein programming comprises channel +FNinjection.
 9. The method according to claim 1, wherein the memory arrayhas a NOR architecture, and wherein programming comprises channelinitiated secondary electron injection.
 10. The method according toclaim 8, wherein programming at least one of the memory cells comprisesapplying a voltage of about +16 V to about +20 V to at least oneselected word line and grounding the substrate.
 11. The method accordingto claim 9, wherein programming the at least one memory cell furthercomprises applying a voltage of about +7 V to at least one non-selectedbit line.